The major goal of this project has been to quantify stages of the HIV- 1 life cycle and understand how replication kinetics is affected by viral and cellular structures. We used a variety of biophysical, cellular and molecular biology techniques combined with mathematical modeling. We developed a mathematical model of HIV-1 infection kinetics in tissue cultures which described quantitatively the virus spread and predicted oscillations in the number of infected cells and virus particles. This prediction has been confirmed experimentally. We also estimated the number of infecting virions produced by one cell in HIV-1 patients. Interestingly, this number (2-3) was about the same for patients with primary infections and AIDS patients, which indicates a lack of significant immune response to HIV-1 antigens in AIDS patients. We studied quantitatively two major stages of the virus life-cycle - entry and integration. We are developing a new experimental system for a quantitative analysis of HIV-1 binding and fusion based on virus particles produced by recombinant vaccinia virus. We succeeded in optimizing the conditions for labeling of the virus particles with a fluorescent dye (PKH-26) which allowed the accurate measurement of their number by a videoimaging system. We also continued to study the modulation of the HIV-1 co-receptor molecule(s) by phorbol esters. It was found that phorbol esters can induce down modulation of tailless CD4 after binding to gp120, most likely due to down modulation of the co-receptor molecules. We continued to study the effect of the HIV-1 U5 3' end DNA on integration. Our working hypothesis is that the fine 3D structure of the HIV-1 termini is critical for the efficiency of integration. A NMR study to determine the structure of DNA fragments from the HIV-1 U5 3' end is now in progress. These findings have implications for understanding the mechanisms controlling the HIV-1 life-cycle and the development of AIDS, and for a rationale design of antiviral drugs.